This invention relates to a method of using a drug to treat patients who undergo a surgical operation called "coronary artery bypass grafting" (abbreviated herein as CABG). In this type of surgery, segments of coronary arteries are replaced after they have become clogged or blocked (occluded) by cholesterol or fat deposits, to an extent where they cannot be adequately reopened by less invasive techniques such as balloon angioplasty.
When replacement or bypass of the artery segments becomes necessary, the chest is opened and the clogged segments of the arteries are bypassed, using blood vessel segments harvested from elsewhere in the patient's body, usually from a saphenous veins in the patient's legs and/or a mammary vein in the patient's chest.
Since the original coronary artery segments are usually left in place (empty, and not carrying blood) on the surface of the heart, this procedure is often referred to as an artery bypass, rather than an artery replacement. As used herein, terms such as "artery bypass" and "artery replacement" refer solely to coronary arteries, and are used interchangeably, regardless of whether a native artery segment is left in place or removed from the heart.
Other procedures (such as balloon angioplasty) and drug treatments (such as cholesterol-lowering drugs) have been and are being developed to provide patients with other options, and open-chest surgery generally is not used unless such other procedures are deemed inadequate for a specific patient. Nevertheless, CABG surgery remains very common, and is described in more detail in medical texts such as Gibbon's Surgery of the Chest (Sabiston and Spencer, eds., Saunders Publ., Philadelphia, Pa.).
During CABG surgery, the heart is usually stopped from beating while the arteries are being worked on, so that the surgeons will not have to perform delicate surgery on a "moving target". Normally, the heartbeat is stopped by perfusing the heart with a liquid called "cardioplegia" solution, which contains a high concentration of potassium to block the electrochemical cellular interactions that initiate and control each heart contraction.
While the heart is not beating, most of the patient's body (excluding the heart and, to some extent, the lungs) are supported by a cardiopulmonary bypass machine (also called a CPB machine, or a heart-lung machine). This machine receives deoxygenated blood from the patient's body, adds oxygen and various nutrients to the blood, and pumps the oxygenated blood back into the patient's body, excluding the heart.
All references herein to the word "bypass" by itself, or to phrases such as circulatory bypass, cardiopulmonary bypass, or CPB (i.e., any and all references to "bypass" that do not refer explicitly to "coronary artery bypass grafting" or "CABG"), are intended to refer to circulatory bypass of the heart and lungs, i.e., the condition wherein the heartbeat is stopped for the purposes of the surgery, and blood supply to the brain and the remainder of the body, excluding the heart and lungs, is provided by an extracorporeal machine that oxygenates and pumps the blood.
During this type of bypass, the body and brain are cooled several degrees, to reduce the need of brain cells and other cells for oxygen and nutrients. The heart muscle must be chilled to a substantially colder temperature, since (in most operations) the heart receives no oxygenated blood whatever from the bypass machine. Most commonly, the chilling procedure involves pouring ice-cold saline slush directly onto the heart, and allowing the cold liquid to remain in contact with the heart for a while, in a basin that is formed when the pericardial sac is cut open to expose the heart.
While a patient is on bypass, the heart suffers from a condition called "ischemia". Ischemia refers to inadequate blood supply to an organ. Since blood is the only source of oxygen and nutrients for cells and tissue, ischemia imposes a major stress on cells and tissues. This is especially true for the heart and brain, which are much more vulnerable to ischemic damage than any other organs, because of various physiological factors. If ischemia persists in either of these organs for more than a few minutes without chilling the organ, severe metabolic derangement can begin to occur, and it can lead quickly to large-scale cell death, and to tissue death ("infarction") which can rapidly become lethal to the patient.
After the CABG portion of the surgery is completed, the surgeons flush out the potassium-containing cardioplegia liquid from the heart, and rewarm the heart muscle by passing warm blood or other liquid through the coronary arteries and veins. As the heart warms up, it usually begins to fibrillate, and the surgeons use electrodes to defibrillate the heart and restart the heartbeat.
When surgeons try to restart the heart after a period of cardiopulmonary bypass, the ischemic insult/damage to the heart muscle may be manifested in various ways. In almost all cases, at least some aberrations (including cardiac arrhythmias, abnormally rapid or slow heartbeat, ventricular fibrillation, or diminished pumping capacity) are likely to arise in varying degrees. These aberrations in heart performance, triggered by the ischemic and surgical insults to the heart, can trigger various complex interactions with and within the heart tissue that is still trying to recover from the ischemic period, in ways that tend to further complicate and aggravate hemodynamic (blood circulation) insufficiency during the period shortly after surgery.
To help understand the very complex challenge facing patients and surgeons involved in these procedures, it should be kept in mind that open-chest surgery is not done on healthy and vigorous patients with enough reserve capacity to help them withstand a major assault. Instead, it is almost always done on badly-weakened patients who have been struggling with serious heart problems for years, and whose health has slowly deteriorated to a point where they can no longer lead even a semblance of normal life without a major, life-threatening surgical intervention.
With proper care, the cardiac pumping aberrations that usually arise when the heart is restarted after bypass usually diminish within a few hours or days, as the patient gradually recovers from the operation. Nevertheless, these abnormalities are themselves a form of stress, and they make it more difficult and time-consuming for a patient who has been through CPB surgery to fully recover. In addition, these aberrations never completely disappear in some patients. Those patients must live with such problems, as both a symptom and a source of stress on their hearts, for the rest of their lives, which are often seriously degraded and shortened by the lasting and lingering damage to their heart's ability to function and pump blood normally and properly.
In addition, in a substantial fraction of CPB operations, the heart fails to begin beating properly in response to normal stimuli, while the chest remains open and the patient is still on the operating table. This type of crisis immediately becomes an all-out emergency, and the surgeons must rapidly resort to more powerful (and potentially damaging) stimulant drugs, and to electrical stimulation of the heart using higher voltages. In some cases, temporary implantation of a left-ventricle assisting pump or an aortic balloon pump becomes necessary; either of these can help handle some of the pumping load while the heart tries to recover some of its strength, but such devices are accompanied by other problems.
These emergency measures, if required to overcome a life-threatening crisis, impose even more stress on the patient's heart and body, and if these measures don't succeed quickly, the patient may die or suffer severe and permanent brain damage, comparable to a massive stroke. Roughly 3 to 10 percent of the patients who undergo CABG surgery never fully recover from the operation. This percentage varies, depending on factors such as the age and condition of a patient, which coronary arteries were affected, how badly each specific artery was affected, and the skill of the surgeons and anesthesiologists who performed the operation.
HEMODYNAMIC MEASURES OF HEART DAMAGE AND HEART RECOVERY
There are various ways to measure the condition and performance of the heart, using mechanical pumping performance ("hemodynamic") criteria, as well as biochemical criteria involving blood chemistry. These values can be used to measure how must stress and damage was inflicted on a heart during CPB surgery, and how well and quickly the hearts were able to recover from the ischemic stress of the surgery. Several of the most commonly used measuring techniques are briefly summarized below, since they were used to show that certain treatments disclosed herein, involving fructose-1,6-diphosphate (FDP), enabled patients to recover from CPB surgery in a better, faster, and healthier manner than untreated control patients who did not receive FDP treatment.
Hemodynamic indicators are evaluated by recording the pressures generated in certain parts of the heart or blood vessels, and by measuring or calculating volumes of blood being pumped (or certain other indices of performance) by any of the four chambers of the heart. One mechanical measurement that is relevant herein is called "pulmonary artery wedge pressure" (PAWP). This is used as an indicator of pressures inside the pulmonary circulation, which provides insight into how well the left side of the heart is functioning. A catheter is commonly inserted into the main pulmonary artery, and the catheter tip is directed into a small branching pulmonary artery. The tip of the catheter is pushed into the artery until it wedges tightly in place, blocking blood flow through that small artery. A "wedge pressure" can then be measured at the tip of that catheter.
This fluid pressure level (which fluctuates in a cyclic manner, with each heartbeat) will differ from the pressure in the left atrium by a relatively constant amount (usually between 2 and 3 millimeters of mercury column), depending on the condition of the capillaries in the patient's lungs. PAWP values that are higher than normal reflect a certain type of problem wherein blood is "piling up behind the left heart" (i.e., the right ventricle is capable of pumping blood into the lungs in adequate amounts, but the left ventricle of the heart cannot pump that same quantity of blood through the legs, abdomen, arms, head, etc., quickly enough; as a result, blood pressures inside the lungs and pulmonary veins leading back to the heart tend to grow abnormally high). This can help evaluate certain types of heart disease, such as congestive heart failure.
Since the condition of the capillaries and other blood vessels in the lungs should not change substantially over the course of a single surgical procedure, PAWP pressures that are measured before, during, and after surgery can indicate how severely the performance of the left side of the heart was affected by the surgery. Most patients show some increase in PAWP values immediately after bypass surgery, due to factors such as the noxious effects of anesthetic drugs, delays in returning to normal body temperature, and the obligatory ischemic insult to the heart during circulatory bypass. In general, a sharp or large increase in PAWP values during cardiac surgery is a danger sign; a small increase in PAWP values is desirable and indicates a lower level of stress and damage to the heart during the surgery.
Other common ways of measuring a heart's pumping performance include:
1. "left ventricular stroke work index" (LVSWI), which measures how much pumping work the left ventricle of a heart does during each heartbeat. For each patient, this quantity is divided by the surface area of that patient's body when calculating a LVSWI value, in order to make these values comparable between patients of widely varying body sizes and weights.
2. "cardiac output" (abbreviated as CO), which measures how many liters of blood are pumped by the heart per minute.
3. "cardiac index" (CI), which is cardiac output (liters of blood pumped per minute), divided by the body surface area of the patient, to make these values more comparable between patients of varying sizes and weights.
These values (and various other hemodynamic measurements) can be measured as described in various books and articles, such as pages 319-339 of Grossman and Baim 1991. All of these values were measured in the tests disclosed herein, and the resulting data all support the conclusion that FDP injection shortly before bypass begins can substantially improve the heart's pumping performance following the surgery.
BIOCHEMICAL MEASURES: CREATINE KINASE
In addition to hemodynamic measurements, there are also ways to measure damage to the heart, using blood chemistry. When heart muscle cells die, the repeated contractions of the heart muscle caused by the heartbeat causes the internal contents of the dead heart cells to be squeezed and forced out of the cell membranes, fairly rapidly. This causes certain enzymes that are normally kept inside viable cells to be squeezed out from the dead cells, and those enzymes enter the circulating blood.
One enzyme that is widely used to evaluate cardiac patients is creatine kinase (CK), a large protein that cannot permeate out of a cell while the cell is alive. Since CK enzymes cannot escape from heart cells that are still living and viable, the concentration of CK enzyme that has escaped from intact cells and entered the circulating blood of a cardiac patient provides a standard method of evaluating the extent of heart cell death and permanent tissue damage in that patient, and helps distinguish between temporary symptoms (such as angina) and permanent damage.
Accordingly, if a candidate drug can reduce CK levels in blood during and after surgery involving cardiopulmonary bypass, the reduction in blood CK levels indicates that the candidate drug helped protect the heart against cell death and tissue damage during the ischemic insult created in the heart by the circulatory bypass. As described below, FDP treatment did indeed have this very useful effect.
It should be noted that heart cells contain a dominant version of this enzyme, usually designated as the CK-MB isozyme; other isozymes, usually called the MM and BB isozymes, are found in higher levels in skeletal muscle cells and brain cells. In the initial "Stage 1" tests described below, total CK levels in circulating blood (taken from a forearm vein) were measured. By contrast, more precise tests which measured only the CK-MB isozyme were used in Stages 2 through 5. The results of all of these measurements clearly indicated that FDP treatment does indeed reduce CK levels in circulating blood, which in turn indicates that FDP treatment as disclosed herein can substantially reduce the death and destruction of heart cells and heart tissue, during cardiopulmonary bypass surgery.
ATRIAL FIBRILLATION
In addition to various types of cardiac aberrations (such as abnormally rapid or slow heartbeats, arrhythmias, etc.) that transiently occur in nearly all CPB surgery patients as their hearts are being restarted after bypass, a substantial fraction (usually about 30%) of patients who have chronic heart problems severe enough to require open-chest surgery suffer one or more episodes of atrial fibrillation (hereafter referred to as "A-fib") within several days after surgery.
A-fib is substantially different from ventricular fibrillation; in V-fib, either or both of the ventricular pumping chambers ceases to pump blood, thereby causing complete cardiac arrest, which can kill a patient within a few minutes. By contrast, A-fib does not rise to the level of an immediately life-threatening crisis.
Nevertheless, if A-fib occurs, it is a serious and potentially very dangerous event, for a number of reasons which include: (1) it disrupts and interferes with proper blood flow and transport through the heart, thereby impeding the ability of an already-weakened heart to adequately supply the body with blood; (2) the much-too-frequent and badly uncoordinated electrical impulses that can be emitted by affected atrial tissue can trigger and provoke irregularities in adjacent ventricular tissue, leading to potentially serious ventricular pumping problems such as bradycardia, tachycardia, and arrhythmias; and, (3) because an atrial chamber is not contracting properly, it poses a serious risk of blood stasis inside the affected atrial chamber. In other words, a small quantity of blood may become effectively trapped and completely stationary, in some corner, pocket, or recess inside the atrial chamber.
If blood stasis occurs as a result of A-fib, the non-moving pocket of blood may form a major blood clot, which poses a major threat of becoming dislodged and travelling to the lungs or brain, leading to a major life-threatening stroke or pulmonary embolism. To prevent this type of life-threatening risk from atrial fibrillation, people who suffer from A-fib must be put on an anti-coagulant drug. However, such drugs create their own problems and adverse side effects; among other things, they can delay and retard the ability of the body to repair itself from the cutting and suturing required by the surgery.
Accordingly, even a single episode of A-fib following cardiac surgery is a very important event, which requires the patient to be placed on anti-coagulant drugs, and which also means that the patient will require more extensive and careful monitoring, during recovery from surgery. A single episode of A-fib following cardiac surgery usually extends the length of the hospital stay for that patient by at least 3 days, and quite often up to 5 days longer than normal, and requires higher levels of monitoring and medical attention while the patient remains in the hospital. Because of the high costs of this type of intensified care in a hospital, a single episode of A-fib following cardiac surgery usually increases the cost of a patient's hospital stay by at least $10,000, on average.
Accordingly, atrial fibrillation is a very important factor (also called an "end point" for purposes of statistical analysis) for evaluating the safety and efficacy of any potential drug that might be useful for treating patients who undergo cardiac surgery, or any other surgery involving cardiopulmonary bypass.
Atrial fibrillation requires and deserves special attention herein, because it has been discovered, through clinical trials on humans, that the rates and risks of A-fib after CPB surgery reveal an apparently major dividing line between two totally different things. First, certain methods of using FDP before and during CPB surgery have been shown to be safe, effective, and highly beneficial for the heart, and substantially reduce the risks of atrial fibrillation after CPB surgery. But second, other unsafe methods of using FDP in CPB surgery can have the opposite effect, and apparently can substantially increase the risks and occurrence rates of atrial fibrillation after surgery.
Accordingly, the invention disclosed herein relates to a method for intravenously injecting FDP into patients in a dosage and manner (involving factors such as the timing of injections, and the possible co-administration of one or more additional active agents along with the FDP, to counteract a specific danger posed by the FDP) that reduces heart damage during surgery and improves, in all respects, the rate and extent of heart recovery after surgery. By contrast, if the FDP is administered is an unsafe dosage and manner, as also discovered and discussed herein, the risk of A-fib will be substantially increased rather than reduced.
In brief, the data obtained to date from human clinical trials indicates that if FDP is administered in a manner that avoids a substantial accumulation of excess lactic acid (which is one of the main chemical byproducts of administering FDP under oxygen-deficient conditions), the FDP can substantially decrease the risks and occurrence rates of atrial fibrillation. However, if FDP is administered in a dosage and manner that causes or aggravates lactic acidosis, the risks and rates of atrial fibrillation increase, rather than decrease.
PVR AND PULMONARY HYPERTENSION
Another very important factor in CPB surgery involves blood flow through the lungs. This is directly affected by a parameter called "pulmonary vascular resistance" (PVR), which relates to the condition of the lungs, rather than the heart.
Briefly, "pulmonary vascular resistance" (PVR) measures the drop in blood pressure between a pulmonary vein (exiting the lungs) and a pulmonary artery (entering the lungs). This drop in blood pressure is caused by the resistance to fluid flow through blood vessels (mainly the pulmonary capillaries) inside the lungs. High levels of vascular resistance (also called "pulmonary hypertension") indicate that blood is not flowing properly through the lungs, due to problems such as edema, inflammation due to an allergic response, immune response, histamines or cytokines, or other forms of tissue stress or damage inside the lungs.
Since it measures a drop in pressure, PVR can be expressed in metric terms (dynes-second/cm.sup.3), or in terms of millimeters of mercury column. It can also be expressed as a PVR Index (PVRI), in a manner comparable to cardiac index or LVSWI, by dividing a PVR value (the metric version) by the surface area of the body of a patient, to give values in dynes-second/cm.sup.5.
The results of the tests disclosed herein indicated that FDP treatment in proper dosages, before and during CPB surgery, caused a substantial and potentially very useful benefit, by preventing large increases in PVR levels during and after the surgery. These results are described in more detail below. Based on the highly favorable PVR data, it appears that some type of cellular reaction involving FDP apparently occurred inside the lungs, which helped the lungs resist edema, tissue inflammation, and possibly other forms of stress or damage during the ischemic bypass period.
This was a potentially very useful and valuable finding, since pulmonary hypertension is an exceptionally difficult and often intractable problem to treat, and often does not respond to conventional treatments (such as vasodilators and inotropic drugs) that can be used to treat elevated vascular resistance in the remainder of the body. Pulmonary hypertension is a very common and very important contributing factor in deaths following cardiopulmonary bypass surgery; indeed, in almost all deaths that follow shortly after CPB surgery, where a patient never fully recovered from the surgery, pulmonary hypertension is almost always involved, as one of the main causes of death.
This is largely due to the fact that pulmonary hypertension imposes its entire resistive load against the right ventricle of the heart. The right ventricle only pumps blood through the lungs, so it is substantially smaller and less powerful than the left ventricle, which supplies blood to the entire remainder of the body. While the left ventricle is well-adapted to cope with large transient increases in pumping loads, such as caused by exercise and other physical exertions, the right ventricle is not well adapted to handling those types of increased pumping loads. Since pulmonary hypertension following CPB surgery can impose a heavily-increased load on a portion of the heart that is not well-adapted to coping with extra-heavy loads, it is one of the most difficult and dangerous problems that can arise after such surgery.
Accordingly, the discovery that if FDP is injected in properly controlled dosages, before bypass begins, it can substantially reduce elevated PVR levels after CPB surgery, is a potentially very important and very useful discovery. It appears to offer a potentially very important breakthrough in treating a hugely important problem that, until now, has been one of the most difficult, dangerous, and intractable problems in CPB surgery.
OTHER INDICES OF SURGICAL HEART DAMAGE AND RECOVERY
In addition to all of the foregoing, there are three other ways of evaluating whether a candidate treatment can actually help protect the heart against damage during CABG surgery, or any other surgery involving cardiopulmonary bypass.
These three methods of analysis included: (1) measuring how long the patients had to remain in an intensive care unit (ICU) following the surgery, before they could be transferred to ordinary hospital rooms; (2) measuring how much dopamine (an inotropic drug, which temporarily increases the strength of the heartbeat but which also causes dangerous side effects in already-weakened hearts) had to be used to stimulate the hearts of patients after CABG surgery; and, (3) measuring the quantity of vasodilator drugs (which reduce the resistive pressures a heart must overcome in pumping blood through the body, by dilating blood vessels) had to be used to stabilize patients after CABG surgery.
All three of these indicators were measured in the clinical trials described herein. The data from all three measurements indicated that FDP treatment, when properly controlled as to dosage and manner, was highly beneficial to the patients. As shown in FIGS. 8-10, in two separate and independent sets of double-blinded tests, FDP treatment reduced the average time spent in intensive care units, by roughly half; it also reduced the amount of vasodilator drug usage by roughly half; and it also reduced the amount of inotropic drug usage by more than half.
These were outstanding results. When taken together with substantial improvements in hemodynamic performance after surgery, substantial reductions in CK blood levels after surgery, substantial reductions in pulmonary vascular resistance after surgery, and apparently reduced occurrence rates for atrial fibrillation when the FDP dosages were held to levels that did not cause excess lactic acid accumulation, these data clearly and overwhelmingly show that FDP treatments, when properly controlled, offered an entire suite of interrelated advantages, to patients undergoing surgery that required cardiopulmonary bypass.
FAILED PRIOR EFFORTS TO EFFECTIVELY TREAT CABG PATIENTS
Despite the best efforts of thousands of cardiac surgeons and other researchers who have been working for decades to solve or minimize the problems that inevitably accompany open-chest cardiac surgery, many patients who undergo CABG surgery often suffer substantial (and in many cases severe) damage to their heart muscle and tissue, due to the surgery.
Since CABG surgery is so common and so widely used, this is a major medical problem. It has been known about and closely studied for decades, but it has not yet been solved.
The problems of limited and inadequate progress in this field of research are rendered even worse by the fact that tests using lab animals or cell or tissue cultures have been poor and inadequate predictors of success, in numerous methods that have been proposed to improve the outcomes of CABG surgery in human patients. Table 1 lists a number of drugs which showed good promise for potential use during cardiac surgery, based on in vitro tests (i.e., cell culture tests, as
well as tests on intact hearts that had been removed from sacrificed lab animals and kept beating by mechanical support equipment), and on in vivo tests using intact living lab animals. Regrettably, none of those drugs proved to be useful, in human clinical trials.
TABLE 1 __________________________________________________________________________ EXAMPLES OF DRUGS THAT SHOWED PROMISING RESULTS IN CELL CULTURE OR ANIMAL TESTS, BUT CANNOT EFFECTIVELY PROTECT CARDIAC MUSCLE IN HUMANS Class of agent Example references Outcome in Human tests __________________________________________________________________________ Superoxidase (SOD) Werns et al (1988) FAILED mimetics J Cardiovasc Pharmacol 11: 3-44 Acadescine Menasch et al (1995) FAILED J Thor Cardiovasc Surg 110: 1096-1106 Adenosine Fremes et al (1995) FAILED J Thor Cardiovasc Surg 110: 293-301 Polyethyleneglycol-SOD Omar et al (1991) FAILED J Mol Cell Cardiol 23: 149-159 .beta.-adrenergic Lu et al (1989) FAILED blocking drugs Arch Int Pharmacodyn Ther 301: 165-181 Calcium channel Watts et al (1986) FAILED blocking drugs J Mol Cell Cardiol 20: 443-456 PAF antagonists Koltai et al (1989) FAILED (e.g., Ginkolide B) Eur J Pharmacol 164: 293-302 FR76830 Ishibashi et al (1991) FAILED Cardiovasc Res 24: 1008-1012 Nitrobenzylthioinosine Kuzmin et al (1989) FAILED Fiziol Zh 35: 3-9 Nitroglycerine Feng (1996) FAILED Int J Cardiol 55: 265-270. Pyruvate Crestanello et al (1995) FAILED J Surg Res 59: 198-204 __________________________________________________________________________
As shown by that table and the articles cited therein, research in this field has been very active, but it has been littered with failed efforts to improve the outcomes of a type of surgery that is done hundreds of thousands of times, every year. If there were any "obvious" answers to the daunting task of improving the outcomes of CABG surgery, cardiac surgeons would quickly embrace and use those answers. The fact is, there are no such answers which are "obvious" to the surgeons who actually do this type of surgery.
BACKGROUND ON FRUCTOSE-1,6-DIPHOSPHATE (FDP)
The treatment described herein involves a sugar-phosphate molecule called fructose-1,6-diphosphate (abbreviated as FDP). Some articles refer to this molecule as fructose-1,6-biphosphate, or as fructose-1,6-bisphosphate.
Any references herein to FDP or fructose diphosphate refer only to the 1,6-isomer of fructose diphosphate, with phosphate groups bonded to the #1 and #6 carbon atoms of the fructose molecule. Other isomers (such as fructose-2,6-diphosphate) also occur, but they are of no interest herein.
FDP (the 1,6-isomer) is a naturally occurring molecule which is created and then quickly consumed during a series of chemical reactions inside cells called glycolysis. Since FDP is a short-lived intermediate that is quickly consumed by subsequent reactions, it normally is present in cells only at relatively low concentrations.
Glycolysis is a fundamental biological process that is essential to the generation and use of energy by cells; briefly, it is the process by which glucose, a sugar molecule, is chemically broken apart, to release energy.
In a first set of reactions, which can occur without requiring any oxygen, a molecule of glucose (with 6 carbon atoms) is broken apart to form two molecules of pyruvate, with 3 carbon atoms. These reactions are called the Embden-Meyerhof pathway, and they yield a relatively small amount of ATP (adenosine triphosphate, a high-energy metabolite that is then used to drive other chemical reactions).
Subsequently, either of two things can happen to pyruvate molecules that are formed by splitting apart glucose. If enough oxygen is present in the cells, pyruvate will be oxidized all the way to carbon dioxide and water, in a set of reactions called the Krebs pathway, or the "aerobic" pathway. These reactions release a great deal of energy. However, if not enough oxygen is present in a cell (as occurs under conditions of ischemia, where the blood supply to the tissue has been disrupted), the pyruvate molecules (with 3 carbon atoms) are merely rearranged to form lactic acid, which also contains 3 carbon atoms. This reaction generates no ATP.
Glycolysis is discussed in numerous texts on biochemistry, physiology, or cell biology. For example, any edition of Stryer's Biochemistry, Lehninger's Biochemistry, Guyton's Medical Physiology, or Alberts et al, Molecular Biology of the Cell contains a fairly extensive analysis of glycolysis.
FDP stands at the absolute peak of the energy curve that can be used to show the progress of glycolysis through its various intermediates, starting with glucose and leading to pyruvate. Two molecules of ATP must be consumed, in order to get the process completely primed by "boosting" glucose up to an even higher level of energy contained in FDP. After the peak energy level of the FDP intermediate is reached, the subsequent reactions begin to release that energy stored in the FDP.
Since researchers have known for decades that FDP stands at the very highest peak of the energy curve in glycolysis, numerous researchers have wondered and speculated for decades about whether FDP might be useful, as a drug, to help temporarily boost energy supplies in the cells or tissue of patients suffering from ischemic crises or other problems of ischemia or hypoxia. Any number of scientific articles and patents have been published, suggesting that FDP might be able to reduce cell death and tissue damage, if administered to patients suffering from ischemia or hypoxia. Examples of such articles, which stretch back to at least 1980, include Markov et al 1980, 1986, and 1987, Brunswick et al 1982, Granot et al 1985, Farias et al 1986, Grandi et al 1988, Zhang et al 1988, Cacioli et al 1988, and Lazzarino et al 1989 and 1992. These are just a few examples, and numerous other similar articles are also available. Relevant U.S. patents include U.S. Pat. Nos. 4,546,095 (Markov 1985), 4,703,040 (Markov 1987), and 4,757,052 (Markov 1988).
However, despite these numerous articles and patents stretching back two decades, FDP simply is not used by surgeons to treat patients who are undergoing cardiac surgery, even though every surgeon in the world is well aware of the need for ways to reduce the ischemic stress and damage that is inflicted on hearts during surgery that requires cardiopulmonary bypass.
Indeed, FDP is not used or prescribed by physicians or surgeons for any medical purposes at all, except for a few researchers who are carrying out small-scale clinical trials, none of which (to the best of the Applicant's knowledge and belief) involve cardiopulmonary bypass surgery, except for the tests described below, which were sponsored and funded by the assignee and applicant, Cypros Pharmaceutical Corporation.
In point of fact, medical-grade FDP (i.e., FDP in a form that is suitable for injection into humans, as distinct from the non-sterile chemical, which is available in bulk but which would be completely unsuited and illegal for injection into humans) is not even available in the United States, or in any other industrialized nation with the possible exceptions of Italy and China. Except possibly in Italy and/or China, it cannot be purchased and used on human patients by physicians or surgeons at all, unless the physicians or surgeons go to the extraordinary trouble of developing an entire research project involving FDP as an experimental drug. Any such research project would need to be individually approved as a form of experimentation, rather than treatment, by institutional review boards as well as the federal Food and Drug Administration.
This is the actual and current medical status of FDP. It simply is not used today for CABG surgery, or any other type of surgery or medical treatment, except in a few small clinical trials, which are experimental research rather than a recognized form of medical treatment.
The failure or refusal of surgeons to use FDP on patients who are preparing to undergo surgery that involves cardiopulmonary bypass is believed to be mainly due to at least two major and hugely important factors.
The first major factor which teaches away from the use of FDP to treat cardiac surgery patients is this: since FDP is a diphosphate with a strong negative charge, it is widely assumed by doctors and researchers that it will not enter cells in significant quantities. Since glycolysis occurs solely inside cells, reports which openly state that FDP will not reach cell interiors in significant quantities would appear to pose a major and unavoidable barrier to the successful medical use of FDP. As one example, Pasque et al 1984, a review article, offers a detailed analysis of the presumed and apparent shortcomings of FDP. It reviewed numerous drug strategies that had been proposed for increasing ATP levels in heart tissue, and then completely dismissed FDP as a potentially useful treatment. As stated by Pasque et al, "According to its proponents, [FDP] results in an enhanced rate of anaerobic glycolysis . . . there are arguments based on sound data to refute these claims. First, the likelihood of a phosphorylated compound, such as fructose 1,6-diphosphate, crossing the myocardial cell membrane intact is small. Second, the phosphofructokinase reaction, although thought to be rate limiting in normal myocardium, does not limit glycolysis in ischemic myocardium. Limitation instead occurs at the glyceraldehyde-phosphate dehydrogenase step, which is distal to the phosphofructokinase reaction in the glycolytic pathway. Finally, a lack of direct metabolic effect of fructose 1,6-diphosphate on the ischemic dog myocardium has been demonstrated with no evidence of myocardial ATP preservation or lactate elevation" (Pasque et al 1984, page 4).
Other articles which report essentially the same conclusions include Angelos et al 1993 (title: "FDP fails to limit early myocardial infarction size . . .") Eddy et al 1995 (titled: "Lack of a direct metabolic effect of fructose, 1,6-diphosphate in ischemic myocardium") and Tortosa et al 1992 (title: "Fructose-1,6-bisphosphate fails to ameliorate delayed neuronal death . . .") All of these articles directly contradict the various hypotheses and proposals saying that FDP might be helpful in treating ischemia or hypoxia.
The second major factor which teaches directly away from the possible use of FDP, to treat patients suffering from ischemic or hypoxic crises, involves the fact that FDP, if metabolized by ischemic cells, leads to the production of lactic acid, which can inactivate a crucially important enzyme called phosphofructokinase (abbreviated as PFK)
In glycolysis, the PFK enzyme converts fructose monophosphate into fructose-1,6-diphosphate, by adding a phosphate group to the monophosphate compound. Because of how the glycolytic pathway evolved, the reaction which is catalyzed by the PFK enzyme became the limiting step that controls the overall rate of glycolysis. This limiting and controlling mechanism is absolutely essential to cells and tissue, because it prevents cells and tissue from burning up their energy supplies too rapidly.
Under conditions of ischemia or hypoxia, excess quantities of lactic acid can inhibit and even "poison" (irreversibly inactivate) the PFK enzyme. If PFK molecules are severely inhibited by accumulating lactic acid, it can create a crucially important bottleneck which can shut down the entire process of glycolysis in affected cells. Because the PFK-controlled step plays a crucial role in controlling and limiting energy generation by cells, inactivation of PFK by lactic acid can completely shut down subsequent glycolysis in ischemic cells, thereby stopping all energy production in those cells, and thereby causing the death of those affected cells and tissue.
Several other factors should also be recognized, in evaluating the obstacles that FDP would need to overcome in order to be useful in ischemic tissue.
First, only a small amount of energy is released under oxygen-starved conditions, when glucose (or an exogenous supply of FDP) is broken apart into pyruvate and the pyruvate is then converted into lactic acid. The energy yield of glycolysis under anaerobic conditions is only 47 kilocalories of energy per mole of glucose converted to lactic acid. By comparison, the energy yield of glycolysis under normal conditions, when oxygen is present, is almost 15 times greater (i.e., 686 kilocalories of energy per mole of glucose.
The fact that only very small quantities of useful energy will by released by converting glucose or FDP into lactic acid, under anaerobic conditions, is aggravated by the fact that large quantities of lactic acid will be generated from FDP, if the FDP can somehow enter ischemic cells. Lazzarino et al 1992 described a test where FDP was added to intact isolated hearts, which had been removed from sacrificed lab animals and which were being perfused on mechanical pumping equipment, using perfusion solutions that contained plenty of oxygen. Despite the fact that the perfusion fluid contained plenty of oxygen, only about 10% of the FDP was oxidized all the way to carbon dioxide and water. Nearly 90% of the FDP was converted into lactic acid, which, as noted above, poses a major risk of inhibiting or even shutting down glycolysis by inhibiting the crucial PFK enzyme.
Accordingly, researchers who understand the complexities of glycolysis (including (i) the large amount of lactic acid that is formed when FDP is metabolized under ischemic conditions; (ii) the small amount of useful energy that is generated when lactic acid is formed from glucose or from exogenous FDP; and (iii) the threat that excess lactic acid will inhibit or shut down the glycolysis-controlling PFK enzyme) would assume that the risks of injecting FDP into a patient suffering from an ischemic or hypoxic crisis are high, especially when compared to the small potential benefits, in terms of only low amounts of energy being released when lactic acid is formed from the FDP. Apparently, in the judgment of most researchers and surgeons during the past 20 years, the inability of FDP to enter intact cells, and the risk that exogenous FDP poses of suppressing or even shutting down glycolysis by leading to the generating of excess lactic acid, have outweighed their assessment of any potential benefits FDP might be able to offer, in patients undergoing surgery that requires cardiopulmonary bypass.
The fact that cardiac surgeons (who are acutely aware of the ischemic damage that arises in heart tissue during cardiopulmonary bypass surgery) have not chosen to use FDP, during their surgery on actual patients, directly contradicts and refutes any presumption or assertion that it would be "obvious to anyone with ordinary skill in the art" to use FDP on patients undergoing cardiac surgery. Little or no effort has ever been devoted by any pharmaceutical companies, other than the assignee company herein, to actually developing FDP as a drug, and making it available to doctors who would like to use it in human patients. Under the laws enforced by the U.S. Food and Drug Administration, FDP cannot be sold in the United States for administration to humans, by physicians. With possible minor exceptions in China and Italy, FDP simply is not being administered to patients, by physicians or surgeons, for any type of medical use, except for certain types of experimental testing in small-scale clinical trials.
PRIOR ART PREPARATIONS OF FDP
FDP is sold in bulk, as a non-sterile chemical, for non-medical uses, by chemical supply companies such as Boehringer-Mannheim, located in Germany. Such non-sterile bulk preparations are not intended, and not suited, for injection into humans, and are not included within the term "medical-grade FDP" as used herein.
Currently, the only known preparation of potentially medical-grade FDP which is commercially available anywhere in the world (other than research reagents sold in gram or milligram quantities by specialty chemical companies) is sold in Italy, by a company called Biochemica Foscama. However, the Biochemica Foscama preparation suffers from a number of substantial limitations. It is relatively inhomogeneous, and contains particles of varying different sizes; some appear to be small glass-like beads, while others appear to be relatively sticky, caramelized agglomerations. It is also relatively unstable; while pure FDP is a crystalline white powder, the Biochemica Foscama preparation (especially the beads and agglomerations) turns yellowish-brown within a few weeks, when stored at room temperature, unopened.
The manufacturing method used by Biochemica Foscama is not well-suited for the sterility requirements of human drugs, and the resulting preparation apparently does not have sufficient purity to qualify for sale and use as a human drug in the United States. Briefly, a large tray of a liquid mixture of FDP is frozen, then lyophilized, then ground up into a powder, which is then loaded into vials. It is effectively impossible to ensure sterility when this type of large equipment is used. In addition, FDP is chemically unstable; either of the phosphate groups can spontaneously break off from the molecule, leaving the monophosphate residue, which is effectively worthless. Therefore, a "terminal sterilization" step (such as pasteurizing, autoclaving, or irradiating the FDP after it has been loaded into the vials) cannot be used, because such steps would seriously degrade the resulting chemical.
The Biochemica Foscama company is aware of these shortcomings. However, it apparently has no intent to develop a different and improved manufacturing processes, in view of the general lack of interest in FDP among physicians and surgeons in industrialized nations.
Other dried FDP preparations have been made in Japan and China, as disclosed in U.S. Pat. No. 5,094,947 (issued in 1992 to Nakajima et al, based on a prior Japanese application) and Chinese patents 1,089,615; 1,089,616; and 1,089,654, invented by Ou-Yang et al). However, those research efforts apparently have not been developed further, and to the best of Applicant's knowledge and belief, no effort has been made by either of those Japanese or Chinese research teams to develop FDP as a commercial product, or to obtain approval to sell either of those preparations for use in human patients, either in Japan or China, or in the United States.
Despite the lack of interest in FDP among other drug companies, the assignee and applicant herein (Cypros Pharmaceutical Corporation, located in Carlsbad, Calif.) has invested millions of dollars in clinical trials, to evaluate FDP for use in treating several otherwise intractable medical problems, such as treating sickle cell anemia patients during sickling crises, and for reducing cardiac damage caused by CABG surgery.
In addition, Cypros has also invested a great deal of money and effort in developing a new, improved, and different method of manufacturing a highly pure and sterile form of FDP, which has sufficient chemical stability to provide a shelf life of months and possibly even years, without requiring refrigeration. Unlike any other prior efforts by any other company or researcher, this new method uses sterilizing and manufacturing techniques that are carefully selected and designed to create a completely sterile and stable formulation, in a sealed vial. This new manufacturing method is disclosed in detail in U.S. Pat. No. 5,731,291 (Sullivan and Marangos, 1998), the contents of which are hereby incorporated by reference.
U.S. Pat. No. 5,731,291 was issued based on patent application Ser. No. 08/705,773, which was filed on Aug. 30, 1996, and which was and is owned by the same Applicant/Assignee company that owns this current application.
The discovery that FDP could be useful as a drug for treating patients undergoing CABG surgery predates the August, 1996 filing date of the Ser. No. 08/705,773 application, and was first disclosed in copending U.S. application Ser. No. 08/647,206, first filed on May 9, 1996, entitled "Protection of Patients During Coronary Artery Bypass Surgery Using Fructose-1,6-Diphosphate".
Accordingly, one object of this current invention is to disclose a method of treating patients who are about to undergo coronary artery bypass grafting (CABG) surgery, or any type of surgery that requires cardiopulmonary circulatory bypass. This method involves injecting FDP into such patients, while the heart is still beating before cardiopulmonary bypass begins, in a quantity that reduces the stress and damage inflicted on the heart during the ischemic period of cardiopulmonary bypass, during which the heart muscle is being bypassed and is not receiving fresh blood. This method further requires that the FDP be administered to such patients in a manner which carefully avoids creating lactic acidosis in such patients, during the period immediately following surgery, by methods such as (i) limiting and controlling the amount of FDP in such patients while monitoring their blood pH, and/or (ii) co-administering along with the FDP a second agent that combats and controls lactic acidosis, such as sodium bicarbonate or a drug called dichloroacetate.
Another object of this invention is to disclose a method of infusing patients who are about to undergo CABG surgery with FDP, in a manner which causes the FDP to permeate into the heart muscle while the heart is still beating, using a sustained infusion of FDP rather than a single bolus injection, without causing lactic acidosis in the patient during the period following the surgery.
Another object of this invention is to disclose that pre-bypass infusion of FDP, into patients who are about to undergo coronary artery grafting surgery, can reduce the amount of damage suffered by the heart during surgery, if the FDP is administered to the patient in a manner that does not cause lactic acidosis during the period following the surgery.
These and other objects of the invention will become more apparent through the following summary, drawings, and description of the preferred embodiments.